Accommodation Cues Research Articles

Influence of target color and vergence of light on ocular accommodation during binocular fixation

In an earlier study we reported large discrepancies between ocular refraction and perception of blur for black/white letters. In the present study we report on the influence of target color and vergence of light on the human focusing system. Twenty visually normal volunteers between 14 and 25 years of age participated in this experiment. Targets were brightness-matched red, yellow, green, blue or white 21 minarc letters displayed on the black background of a high resolution RGB monitor, at a 40 cm observation distance. Changes in the vergence of light were effected via increasing amounts of positive and negative power spherical lenses placed binocularly in front of the subject's eyes. Our results showed that: 1) the accommodative level varied as a function of the color of a target, 2) the vergence of light was not an infallible cue for accurate accommodation and 3) the inter-subject variability seen in response to the vergence of light was not linked with the chromaticity of test targets. All subjects showed a decreasing ability to fully relax accommodation with increasing plus power lenses. For minus power lenses, accommodative response profiles were divided into 3 categories, category 1 having accurate accommodation for the test target, category 2 showing a low level lag of accommodation and category 3 being totally unresponsive to increasing divergence of light. While subjects across response categories showed differential endpoints in clinical estimates of positive relative accommodation, other clinical measurements related to refractive error, accommodative amplitude and fusional ability failed to predict the accommodative behavior of our subjects as did measurements of the resting focus of accommodation.

Facilitation of vergence changes by saccades: influences of misfocused images and of disparity stimuli in man.

When confronted with randomly presented targets demanding change in mean visual direction (version of 4.5-7 deg) as well as a change in vergence (1.5-2.5 deg) both naive and experienced subjects can make short-latency saccades which differ markedly and appropriately in the excursion of the two eyes, and which thereby achieve a large fraction of the required change in vergence. Mean values for the portion of vergence change occurring during the saccades, over all target positions, ranged from 41 to 70% for the five subjects tested, but subject-specific directional biases in performance were also conspicuous. When such targets were presented randomly during monocular viewing, so that only accommodation cues could induce vergence change, a consistent fraction (13-48%) of the resulting vergence movement also occurred during the saccades. These data indicate that on average about one-quarter of the intra-saccadic vergence change achieved during binocular viewing could have been due to accommodation stimuli perceived by one eye. The monocular results indicate that the direction of image misfocus (nearer vs. farther), which guides accommodation-vergence, can be correctly distinguished for targets which are about 6 deg from the centre of the fovea, and that this assessment can be made within the 200-300 ms latency for saccadic eye movements, and hence without trial-and-error refocusing. The binocular inequality of saccades during monocular viewing involved conspicuous subject-specific right-left asymmetries, as well as consistently positive within-subject correlations between intra-saccadic vergence change and finally realized accommodation-vergence movement, and consistent correlations between phoria before the saccade and intra-saccadic vergence change.

Binocular cues in the perception of distance of a point source of light.

The ability to make egocentric distance estimates of a single point source of light, seen in darkness and without the cues of changing size and luminance, was investigated in sixteen observers. The attenuation required to maintain constant luminance, when the target was viewed from different distances, was shown to follow the inverse square law providing the angle subtended by the light was less than 20 s arc. Distance changes were also simulated by means of a split mirror which produced vergence cues, or by test lenses to provide accommodation cues. Over the range 0.5 to 9.2 m distance estimates were surprisingly accurate, although there was some overestimation of near and underestimation of far distances. Most observers made good judgements when only convergence cues were varied, whereas no observers made consistently good judgements when only accommodation cues were varied. The difficulties are discussed in terms of the accommodation-convergence link. When distance was simulated by changing convergence and accommodation cues, estimates were not as good as when real distance was changed. Since good estimates were made with brief target exposures, these judgements were not based on subsequent convergence or accommodation changes. It is suggested that the metric or reference against which the apparently absolute judgements were made was the efferent demand signal associated with a 'resting' position of convergence in darkness.

Effectiveness of accommodative aids in reducing empty field myopia in visual search.

When a visual detection task is performed with distant targets in the absence of adequate accommodative cues, a performance loss is obtained which has been attributed to empty field myopia. It is shown that in a visual search situation an accommodative aid located at optical infinity improves detection by approximately 30% over empty field performance. It is further demonstrated that such an aid may overcome the conflicting accommodative cues provided by proximal contours defining the search area, i.e., a situation that is analogous to the detection of distant targets by observers searching through aircraft cabin windows.

Chameleons use accommodation cues to judge distance.

ANY animal that adjusts the focus of its eye lens has monocular distance information potentially available from its focusing mechanism. Such accommodation cues are independent of size and context, and thus could provide an objective measure of distance. Ingle has proposed that such information is used by monocular frogs1 and toads2, both of which he has shown to be capable of estimating distance with one eye3. There is no direct evidence, however, that any animal perceives distance as a function of the plane of focus of the image. The experiments described here demonstrate that chameleons do so in estimating the distance of their prey.

Stereopsis in toads

HARKNESS'S demonstration1 that chameleons rely primarily upon accommodative cues to judge distance encouraged me to examine the mechanisms of depth vision in an amphibian (Bufo marius). Unlike chameleons which have highly mobile eyes, toads make neither convergent nor fixating eye movements, so that, except for stabilising reflexes, their eyes are almost locked in their sockets2. For simple forms of stereoscopic range finding it is an advantage to have immobile eyes because disparity measurements can be converted directly into estimates of absolute depth (Fig. 1a). One might therefore expect toads to use retinal disparity in judging the distance of their prey. Toads have the essential apparatus for stereopsis: there is a substantial binocular overlap both in front of and above the animal3, the interocular distance is relatively large, and neural pathways bring information from corresponding areas on the two retinae to the same region in both mid-brain4 and thalamic5 visual areas. My results suggest that toads do indeed make disparity measurements and should be added to the growing list of vertebrates known to achieve stereopsis (monkey6, cat7, owl8). In addition, toads have a mechanism for measuring depth within their extensive monocular field. Ingle has shown9 that frogs and toads, like chameleons, can estimate the distance of prey almost as well with one eye as with two, and the present data suggest that they do this by monitoring the accommodative state of their eyes when the prey is in optimum focus, as Ingle originally proposed10.